Glass-ceramic articles have been marketed commercially for over 30 years. The initial disclosure of such articles is found in U.S. Pat. No. 2,920,961 (Stookey) wherein it discloses the preparation of glass-ceramic articles through the heat treatment of precursor glass bodies. As explained therein, such articles are prepared in three general steps: (1) a glass forming batch, customarily containing a nucleating agent, is melted; (2) that melt is simultaneously cooled to a temperature below the glass's transformation range and a glass article is shaped therefrom; and, (3) that glass article is heat treated at temperatures above the annealing point of the glass and, frequently, above the softening point of the glass for a sufficient length of time to cause the glass to crystallize in situ. The heat treatment can be scheduled in such a manner as to control the size and, in some instances, the identity of the crystals developed. Therefore, in sum, the crystallization present in a glass-ceramic article can be the result of both the base composition of the precursor glass and the heat treatment the glass body is subjected to.
Glass-ceramic articles containing a spinel-type crystal phase are well known in the art; the original disclosure occurring in U.S. Pat. No. 3,268,3 15 (Stookey). This patent discloses a method for forming a glass-ceramic material consisting essentially, expressed in terms of weight percent on the oxide basis, of 40-70% SiO.sub.2, 14-34% Al.sub.2 O.sub.3, 8-27% MgO, and 0.4-2.5% Cr.sub.2 O.sub.3, with the total of these constituents constituting at least 95% by weight of the glass material, and subsequent glass-ceramic. The patent further discloses that the glass-ceramic exhibits a crystallization consisting essentially of at least one crystal phase selected from the group consisting of a magnesium metasilicate and a spinel.
U.S. Pat. No. 3,873,329 (Beall) relates to the production of hard, high modulus glass-ceramic articles having a composition consisting essentially, expressed in terms of weight percent on the oxide basis, of 44-58% SiO.sub.2, 14-28% Al.sub.2 O.sub.3, 0.5-3.5% B.sub.2 O.sub.3, 10-16% MgO, 3-9% TiO.sub.2 and 3-7% ZrO.sub.2, wherein the TiO.sub.2 +ZrO.sub.2 totals about between 8-13%. The patent discloses a crystal phase assemblage comprised predominately of alpha-quartz and sapphirine with, frequently, a spinel phase (MgO.A.sub.2 O.sub.3) included also. Additionally, minor amounts of such other crystals as rutile, cordierite, zircon, and tetragonal zirconia or zirconium titanate are also formed in the glass-ceramic articles. Furthermore, the patent describes the crystals as fine-grained, homogeneously dispersed, but randomly oriented, within a very minor amount of residual glass (normally less than about 25% by volume). The fine-grained crystals described therein are disclosed as essentially being smaller than 5 microns, with the vast bulk thereof being less than 1 micron in diameter.
U.S. Pat. No. 3,936,287 (Beall et al.), an improvement of the aforementioned Beall patent, describes a method for making a glass-ceramic article exhibiting a modulus of rupture in excess of about 150,000 psi and demonstrating frangibility when fractured, consisting essentially, by weight percent on the oxide basis, of about 48-51% SiO.sub.2, 23-26% Al.sub.2 O.sub.3, 1-3% B.sub.2 O.sub.3, 13-15% MgO, 4-8% TiO.sub.2 and 3-7% ZrO.sub.2, wherein the TiO.sub.2 +ZrO.sub.2 totals about between 9-13%. The method includes a step which consists of contacting the glass-ceramic article with a source of lithium ions in order to replace at least a pan of the magnesium ions to produce crystals of lithium stuffed beta-quartz in the surface layer of the glass-ceramic. The internal crystal phase assemblage of the glass ceramic mirrors that of the earlier described Beall patent.
Lastly, U.S. Pat. No. 3,681,102 (Beall) discloses the production of transparent glass-ceramic articles which possess a crystal content that is substantially free from silicate-containing phases and is at least 20, but less than 50%, by weight. The crystals are disclosed therein as consisting essentially of zinc spinel and as being substantially all finer than 1000.ANG. in diameter. Additionally, the composition of the article is described as consisting essentially of 55-75% SiO.sub.2, 8-25% Al.sub.2 O.sub.3, 2-20% ZnO and 2-12% ZrO.sub.2.
Other compositionally similar patents generally disclose the existence of a spinel-type crystal phase include U.S. Pat. Nos. 3,585,054 (Karstetter), 3,681,102 (Beall), 3,962,514 (Rittler), 4,059,454 (Reade), 4,867,750 (Pinckney), 5,028,567 (Gotoh et al.), 5,079194 (Jean et al.), and U.K. Pat. No. 1,544,779 (Macmillan et al.).
Although all of the aforementioned disclose the formation of glass-ceramic articles which contain spinel crystals, the instant invention can be differentiated from these references for a number of reasons because the exact compositional range, carefully delineated and disclosed herein, is not taught in any of the references; thus, the resulting phase assemblage and the corresponding physical properties are significantly different.